Cell division is the foundation of life, growth, and reproduction. Mitosis and meiosis, two key processes, serve different purposes but are equally crucial. Mitosis creates identical cells for growth and repair, while meiosis produces diverse gametes for sexual reproduction.

Meiosis is a genetic diversity powerhouse. Through independent assortment and crossing over, it shuffles genetic material, creating unique combinations. This process is vital for evolution and adaptation, ensuring each offspring has a distinct genetic makeup.

Cell Division and Genetic Diversity

Mitosis vs meiosis processes

• Mitosis and meiosis are both types of cell division that serve different purposes and have distinct outcomes
  • Mitosis used for growth, repair, and asexual reproduction (hydra) while meiosis used for sexual reproduction and gamete production (sperm and egg cells)
• Stages of mitosis follow PMAT acronym
  1. Prophase: chromosomes condense, nuclear envelope breaks down, spindle fibers form
  2. Metaphase: chromosomes align at equatorial plate
  3. Anaphase: sister chromatids separate and move towards opposite poles (centromeres)
  4. Telophase: nuclear envelopes reform, chromosomes decondense, cytokinesis occurs
• Stages of meiosis occur in two rounds PMAT I and II
  1. Prophase I: chromosomes condense, homologous chromosomes pair up forming synapsis, crossing over occurs
  2. Metaphase I: homologous chromosome pairs align at equatorial plate
  3. Anaphase I: homologous chromosomes separate and move towards opposite poles
  4. Telophase I: nuclear envelopes reform, chromosomes decondense, cytokinesis occurs
  5. Prophase II: chromosomes condense again
  6. Metaphase II: chromosomes align at equatorial plate
  7. Anaphase II: sister chromatids separate and move towards opposite poles
  8. Telophase II: nuclear envelopes reform, chromosomes decondense, cytokinesis occurs
• Outcomes differ between mitosis and meiosis
  • Mitosis produces two genetically identical daughter cells with same chromosome number as parent cell ($2n$)
  • Meiosis produces four genetically diverse haploid cells ($n$), each with half the chromosome number as parent cell (gametes)

Meiosis for genetic diversity

• Independent assortment occurs during metaphase I of meiosis
  • Homologous chromosome pairs align randomly at equatorial plate
  • Results in different combinations of maternal and paternal chromosomes in resulting gametes (mix and match)
• Crossing over occurs during prophase I of meiosis
  • Homologous chromosomes exchange genetic material through formation of chiasmata
  • Results in new combinations of alleles on chromosomes (swapping segments)
• Combination of independent assortment and crossing over generates vast array of genetically unique gametes
  • Contributes to genetic diversity of offspring in sexually reproducing organisms (humans, animals, plants)

Chromosome segregation importance

• Proper chromosome segregation ensures daughter cells receive correct chromosome number
  • In mitosis, each daughter cell receives complete set of chromosomes ($2n$)
  • In meiosis, each gamete receives half the chromosome number ($n$)
• Errors in chromosome segregation can lead to aneuploidy, an abnormal chromosome number in daughter cells
  • Nondisjunction is common cause of aneuploidy, where chromosomes fail to separate properly during anaphase
• Consequences of aneuploidy include developmental abnormalities, intellectual disabilities, increased miscarriage risk
  • Monosomy: one chromosome missing (Turner syndrome, 45,X)
  • Trisomy: one extra chromosome present (Down syndrome, 47,XX/XY,+21)

Cell Ploidy and Life Cycles

Haploid vs diploid cells

• Haploid cells ($n$) contain one set of chromosomes
  • Produced by meiosis
  • Examples include gametes (egg and sperm cells), fungi (yeast), some algae
• Diploid cells ($2n$) contain two sets of chromosomes, one from each parent
  • Produced by fertilization which fuses haploid gametes
  • Examples include somatic cells in animals (skin, muscle) and plants (leaves, roots)
• Life cycles vary in dominant ploidy stage
  • Haplontic life cycle: dominant haploid stage, brief diploid stage (many fungi and algae)
  • Diplontic life cycle: dominant diploid stage, brief haploid stage (animals and some plants)
  • Alternation of generations: alternating haploid (gametophyte) and diploid (sporophyte) stages (plants and some algae)
    • Gametophyte produces gametes through mitosis (moss, ferns)
    • Sporophyte produces spores through meiosis (pine cones, flowers)